1. Field of the Invention
This invention relates to a beam-index type color television picture tube, and more particularly to an electron beam deflecting device for the picture tube which can eliminate the widening and twisting distortion of the electron beam having an oval cross-section.
2. Description of the Prior Art
A beam-index type color picture tube apparatus of a conventional construction comprises a beam-index picture tube, which includes a phosphor screen provided on the inner surface of the faceplate and a neck portion containing an electron gun generating an electron beam of an oval cross-section, and a deflecting yoke provided outside of the neck portion of the picture tube for deflecting the electron beam.
The phosphor screen comprises red, green, and blue phosphor stripes, light absorbing stripes each disposed between the phosphor stripes. Indexing stripes are provided with interposition of an aluminum layer on the side of the phosphor stripes facing the electron gun in a predetermined relation with the phosphor stripes.
In order to obtain a high-quality image by the beam-index type tube apparatus of the conventional construction, the electron beam should have a cross-section of a predetermined dimension throughout the entire image of the picture tube. However, because of aberrations in the deflecting field the dimension i.e. size and shape of the electron beam is enlarged in the peripheral part of the image surface, with the electron beam rotated in the diagonal end part, thus making it impossible to obtain a high-quality image.
More specifically, in FIG. 4 showing one quarter of the display image X and Y represent a horizontal axis and a vertical axis of the beam-index type picture tube. As will be apparent from FIG. 4, at the central part of the display image, the electron beam has an oval cross-section of a predetermined size. However, at the end part of the horizontal axis X, the width in cross-section of the electron beam is widened, the beam straddling the three color stripes, while at the end part of the vertical axis Y, the length of the cross-section of the electron beam is increased, thus overlapping the adjacent scanning lines. At the ends of the diagonal line of the image the electron beam is rotated or inclined.
In order to prevent the electron beam from being expanded or rotated as described above, a dynamic focus correction has been imparted by applying voltages to the electron gun in synchronism with the horizontal and vertical deflections. However, these voltages amount to several hundred volts, and considering the fact that the static focusing voltage is of the order of several kilovolts, the circuits for supplying the voltages become too much complicated and costly. Particularly the circuit for the dynamic focus correction of the horizontal scanning of a high frequency is much more costly in comparison with the circuit for the dynamic focus correction for the vertical scanning. So, mere elimination of the former circuit would contribute much in the industry related thereto.